The main reason is that it is not that easy to dominate 300 million Americans. If a thousand terrorists each take out 30 of us, that will be a loss of 30 thousand Americans to eliminate 1000 terrorists. It's a high price to pay but not so high that we wouldn't pay it. (We lose more people each year on the highways.)
The attack on 9/11 killed about 3 thousand of us. That was about as well as the terrorists might hope to do. Since that time we have killed far more than 3 thousand Al Qaeda, Taliban, and other sympathizers, I'm sure.
From a purely tactical standpoint, I would rather be one of us than one of them. What we must not tolerate is any government attempt to disarm us further in a "misguided" effort to obtain security.
The fear that terrorists inspire has a disproportionate impact to the actual numbers killed, it has always been thus. So it still mystifies me why they haven’t tried it yet...
I think if such terrosist fire-arms attacks on business centers happened, it would be an attempt to hurt the economy. Maybe they think there’s not much point in this economic climate. But before the economy really tanked? It seems weird that it didn’t happen, at least to me.
Another point about why it might not be done is that almost 10 years after 9/11, we have the weakest and most islamicist sympathetic regime we have had in decades, that is hell bent an weakening the US as much as it can. Why should they attack when they basically have what they want, and we seem to beating ourselves much better than they could? Maybe they think it is better not to rile us, just let us defeat ourselves.
Freegards
Maybe Plan B was to get a sympathizer in office and use him to back the rise of this Ummah, as Obama seems to be doing.